Cannabis Seeds Physiology

Seeds sit at the roots of (cannabis)cultivation and propagation. At the base of every plant, and the beginning of every clone, sits a seed.

In Nature mechanisms of seed dispersal, seed dormancy and seed development are a habitual affair, governed by the seasons and individual microclimates. As horticulturalist the scenario isn’t always so simple.

Cultivated seeds tend to come in paper packets, the origin and treatment of which often remains unknown by the grower. These seeds are, after-all, procreated by hand.

A basic understanding about seed physiology and morphology therefore sits at the foundations of gardening success. In this issue we shall take a brief look at mechanisms of Seed Dispersal, Seed Dormancy, and Seed Development.

Seed Mechanisms

Many of the mechanisms behind the sowing of cannabis seed are often taken for granted by horticulturists. It is normally accepted that seeds are seeds (much and much the same); that one seed-line can/ should be treated as another seed-line, under the same controlled conditions, to the same result. That failure in the garden is the result of a “poor breeder”, a “bad seed packet”, and/or an “infertile seed-line”. Here we hope to highlight the fact that: individual seed-lines, and in some cases individual seed, seek slightly different methods of treatment in order to trigger the best patterns of growth. Seeds are unique. Upon sowing, each seed, no matter how closely related to its siblings, becomes an individual in its own right. Let’s not forget that from seed, seedlings grow into adult plants (each of which has slight if not major differences from the next plant). During the earliest stages of cannabis seed development and germination even the smallest alteration in microclimate can manipulate the physiology of a seeds potential. In this way, under artificial conditions, individual seed (and collectives) alter their metabolism in accordance with their climatic surroundings (the condition beneath which they are kept and sown). To understand these processes we shall consider seed mechanisms against the order that they occur in Nature.

Seed Dispersal

As an annual plant, and as a means of propagation, Cannabis sets-to-seed from female flowers once a year. As the nights get longer, the pollinated (seeded) flowers ripen. At this point seed dormancy begins! The swollen sepals, which are at this stage seed laden, begin to split and the seed are aired. Some cannabis seeds are cast from their shell-pods onto the ground, whilst some seeds remain hidden far deep within the flowers themselves. Whilst related to plant size, the number of seed per plant is often relative to subspecies and microclimate. For example, with lowland species of sativa the number of cannabis seeds per plant often exceeds the number of seed per plant found in highland species of indica. During winter and spring, thawing ice and snow aid the dispersal of highland species over distances, whereas under equatorial climates, monsoon rains aid the dispersal of lowland species over local landscapes. This reflects a need for competition in certain subspecies and/or an inherited mechanism of seed dispersal in others.

Those cannabis seeds that are not disturbed by beast, bird or human return to the earth from whence their origins began. As an inherited mechanism of seed dispersal, animals play an extremely functional role in the dispersal of seed for several reasons. Firstly via digestion animals transport seed into new locations. Whole seed are passed into smelly prefertilised piles of guano/manure. This give seeds a nice home to over-winter in (a place where seed aren’t likely to get eaten by birds). In this way a population of seed can easily find way uphill or across river. This also provides the ideal conditions for dormancy.

Secondly, large herbivore (yak, deer, bison) churn-up the ground surrounding the plants upon which they browse. This creates a perfectly ‘tilled’ environment, upon which new seed can compete with establish weed species. Thirdly, seed-feeding bird hold no-bounds and may carry seed hundreds, if not thousands of miles on the wing.

Finally, humans transport seeds for both food and cultivation. In wild populations, microclimate plays the most important role in seed dispersal by governing the rate of soil morphology itself (the way a soil moves). This dictates the location of seed populations across landscapes and encourages the uptake of natural selection. Seed that descend from higher ground will naturally be washed into lower-ground. Some seed float while others tend to sink (ever wondered why this was?). Seed that readily float will be washed the furthest away from the source of the parent plant by heavy rainfall/snow. Likewise, strong winds aid seed dispersal by literally blowing seed from calyx and stem. Floods, storms, landslides, earthquakes, de-forestation, periods of drought and/or urban development may all contribute towards the process of seed dispersal across any population, within any environment. There are quite literally thousands if not millions of variables involved. Where/when wild populations are subject to human intervention and/or unnatural methods of procreation, the natural ebb and flow of Nature, and natural seed dispersal, is dramatically damaged.

Those warning signs that read – “Please do NOT pick the wild flowers” are there for good reason. And must be perpetuated if wild populations are to remain.

Seed Dormancy

Seed dormancy is a naturally occurring ‘survival factor’ found within many species of plant. Under ‘natural conditions’ the dormancy of cannabis seeds is quite normal. The “over-wintering” of seed is then a naturally occurring phenomenon. That is, when under dormancy (even where perfect germination conditions exist) a seed will NOT germinate! Under “artificial conditions” cultivated plants tend to display lower levels of dormancy. The seed of cultivated plants, especially those used for food, have an unnaturally shallow dormancy period. The ‘survival factor’ has quite literally been bred out of popular cultivar in order to ensure a homogeneous crop/harvest.

There are three different types of seed dormancy – Exogenous Dormancy (external), Endogenous Dormancy (internal), and Combined Dormancy (external and/or internal):

Exogenous Dormancy is governed by the external covering of the seed itself (the seed coat). With Cannabis the seeds coat is relatively thick. The seed is slowly permeable to water via a fissure line in the shell. This mechanical resistance inhibits dormancy. A period of warmth, followed by a period of cold usually encourages germination.

Endogenous Dormancy involves the internal development of the seed embryo (both morphological and physiological). Whilst internal seed morphology is aided by ‘warm stratification’, both small and immature embryos are always found in combination with another type of dormancy (mostly physiological). Physiological endogenous dormancy is the most common type of dormancy found in nature. An inhibitor inside the embryo and/or an incomplete chain of chemical reactions imposes dormancy. Annual species such Cannabis experiences a relatively ‘shallow dormancy’ period. Shallow dormancy can be overcome by “after ripening” (drying seed). A period of ‘cold stratification’ is then required in order for physiological dormancy to be complete. Growers use cold-rooms and refrigerators for this reason.

Combination Dormancy is displayed when several types of dormancy (often internal and external) are displayed in the same seed. This may involve exogenous dormancy (a seed coat), coupled alongside endogenous morphological resistance (embryo development), alongside physiological dormancy (a shallow inhibitor), all within the same seed structure. Cannabis seed display ‘combination dormancy’. The underdeveloped embryos MUST first develop into their full size during a period of ‘warm stratification’ before a period of ‘cold stratification’ can be effective.The best way to test levels of ‘seed dormancy’ against levels of ‘seed viability’ is to play with seeds. Try keeping collectives of marijuana cannabis seeds under different conditions (dry, wet, hot, cold…). Then sow each seed group into separate trays of soil. Place each tray into a contrasting environment (dry, wet, hot, cold…) make note and record the results.

Seed Development

Seed development is a funny old thing… it never ceases to amaze. The smaller the seed, the more amazing the transformation from genetic time capsule into seedling. Let’s face it, this is where the ingrained magic of growing anything begins. What the people in white coats like to call “plant science” is in fact just Mother Nature doing her own thing. Now people may call on the traditionalists, but cannabis seeds like things best in soil. There is just no escaping this. With ‘paper towel’ and ‘glass of water’ methods people can look, touch, and prod the seed as it germinates (and we’d like to think this is one of the best ways to learn about the habit of seeds). Seed development is however an extremely delicate process. For this reason seed development is often perhaps best left untouched by hand.

The germination and viability of seed is foremost governed by patterns of seed dormancy. Only once the natural cycle of embryo development is complete will the germination of seed successfully begin! Fresh cannabis seeds are usually far less viable than those that have been subjected to dry conditions (warm stratification) followed by cool conditions (cold stratification) prior to sowing. Only following a period of dormancy can then true seed germination and accurate measurements of seed viability begin. Before this seedlings tend to be weak.

After thoughts…

Always remember that mechanisms of seed dispersal, seed dormancy, and seed development hold a direct influence over the seeds that grow and the seeds that don’t. Seed may have travelled a long way from the hands of the breeder to the hands of the gardener. Any number of climatic and/or environmental factors may then have already inhibited and/or aided the initial stages of seed dormancy and seed development. It all depends upon the condition(s) beneath which the seed have previously been kept.